As a method of forming a film on a substrate, e.g., a semiconductor wafer (hereinafter referred to as a “wafer”), there is known a method called an ALD (Atomic Layer Deposition) method or an MLD (Multi Layer Deposition) method (which will be collectively referred to as an ALD method), in which a reaction product is deposited by sequentially supplying different kinds of reaction gases reacting with one another to a wafer.
In the ALD method, the internal pressure of a reaction vessel may be changed depending on the kinds of reaction gases supplied to a wafer. For example, if the internal pressure of the reaction vessel is adjusted using a common vacuum exhaust mechanism installed in an exhaust pipe of the reaction vessel, the pressure range may be changed but is restricted by the capacity of the vacuum exhaust mechanism. Thus, depending on the kinds of reaction gases used, it is sometimes impossible to realize an optimal pressure environment.
In the related art, there is available a semiconductor manufacturing apparatus configured to form a SiN (silicon nitride) film by an ALD method. In order to prevent byproducts from being deposited within a vacuum exhaust system, which may be caused by the mixture of two kinds of gases, the semiconductor manufacturing apparatus employs a structure in which an exhaust pipe for an ammonia gas and an exhaust pipe for dichlorosilane (DCS: SiH2Cl2) are installed independently from each other and vacuum pumps are installed in the respective exhaust pipes.
In the semiconductor manufacturing apparatus, an inert gas is supplied when switching the exhaust pipes for the ammonia gas and the DCS gas. The exhaust pipes are switched during the supply of the inert gas. When the exhaust pipes for the ammonia gas and the DCS gas are to be switched, a time period is set so that the inert gas can flow through the exhaust pipes for both the ammonia gas and the DCS gas in parallel during the set time period.
The related art remains silent on the capacity of the vacuum pumps installed in both exhaust pipes in the semiconductor manufacturing apparatus. However, the parallel flow of the inert gas through the two exhaust pipes means that little pressure difference exists between the exhaust pipes and that a gas backflow problem does not exist. In view of this fact, it can be noted that the vacuum pumps installed in the exhaust pipes have the same capacity. Therefore, even if the exhaust pipes are switched depending on the kinds of the reaction gases supplied to a wafer-accommodating process furnace, the range of the internal pressure of the process furnace remains the same as a case where a single vacuum pump is used.